1. Field of the Invention
The present invention relates to a picture signal detecting apparatus for detecting a picture signal from a picture modulated signal wherein a carrier wave has been amplitude-modulated. In particular, the present invention relates to a circuit that suitably operates in a state of overmodulation exceeding 100%.
2. Description of the Related Art
A television receiver down-converts a target radio-frequency television signal received from a broadcast station to a predetermined intermediate frequency (IF), and demodulates a picture signal, color signal, and sound signal from the IF signal. For example, a VIF (video intermediate frequency) circuit demodulates a picture signal including brightness information from an intermediate-frequency picture modulated signal (“picture IF signal” shall be referred to below as “PIF signal”). The PIF signal is amplitude-modulated according to the picture signal, the VIF circuit reproduces a carrier wave of the PIF signal, the reproduced carrier wave is used to perform synchronous detection, and the picture signal is extracted.
FIG. 1 is a block diagram showing a structure of a conventional VIF circuit. A PIF signal input from an input terminal 2 is amplified in an amplifier 4 and then input to a wave detector (VDET) 6 and an APC (automatic phase control) wave detector 8. The APC wave detector 8, a voltage controlled oscillator (VCO) 10, and a phase converter 12 form a phase locked loop (PLL) and reproduce a carrier wave on the basis of the inputted PIF signal.
The phase converter 12 generates two signals having a phase difference of ±45° in relation to the input signal from the VCO 10. One of the signals is output to the APC wave detector 8. The other signal is output to the VDET 6. For example, output having a+45° phase difference is input from the phase converter 12 to the APC wave detector 8. The PLL performs a synchronous control so that a phase difference of 90° will be present between the two input signals input to the APC wave detector 8.
The APC wave detector 8 combines the two input signals, and outputs a current IAPC as a DC component that corresponds to the difference between the frequencies of the input signals. The IAPC corresponds to a degree δ of phase shifting between a target value of 90° and the phase difference between the two signals. A high-frequency component that corresponds to a sum of the frequencies is also output from the APC wave detector 8. An APC filter 14 that is a low pass filter (LPF) is connected to the output side of the APC wave detector 8. The APC filter 14 performs a smoothing operation, and removes the high-frequency component. The APC filter integrates the IAPC at a predetermined time constant, and converts the IAPC to a control voltage for the VCO 10.
For example, when the PLL is configured so that a signal input from the phase converter 12 to the APC wave detector 8 has a phase difference of +90° in relation to the PIF signal input from the amplifier 4 to the APC wave detector 8, a reproduced carrier wave that has the same frequency as the carrier wave of the PIF signal and a phase difference of 0° in relation to the carrier wave of the PIF signal will be obtained as the signal input from the phase converter 12 to the VDET 6. In other words, the phase difference between the PIF signal input to the VDET 6 and the reproduced carrier wave is controlled to be 0°.
The VDET 6 AM-detects the PIF signal from the amplifier 4 by synchronous detection using the reproduced carrier wave from the phase converter 12, and extracts the picture signal. The extracted picture signal is amplified in a video amplifier (VAMP) 16, and output from an output terminal 18.
The picture signal output from the VAMP 16 is input to an automatic gain control (AGC) circuit 20. The AGC circuit 20 controls the gain of the amplifier 4 and adjusts the picture signal to a proper level.
A comparator 22 assesses the overmodulation of the picture signal. The picture signal adjusted to a suitable level is input to one input terminal of the comparator 22. A predetermined standard voltage that corresponds to a threshold value for determining overmodulation is input to the other input terminal. The APC wave detector 8 is configured to control a loop gain of the PLL in accordance with the results of the assessment performed by the comparator 22, as shall be described below.
An upper limit for the overmodulation of the picture is regulated in Japan to be, e.g., a value of 87.5% for ground-based broadcasting. Modulation exceeding this value is referred to as “overmodulation”. Therefore, an overmodulated PIF signal may also be produced in various image media and in the broadcasts of other countries. Problems are presented by a state of overmodulation in that the amplitude of the PIF signal is extremely low and that the PLL will not readily be synchronized. In addition, a problem is presented in that a state of overmodulation that exceeds 100% (referred to below as a “state of strong overmodulation”) the picture signal will be folded back to a region that is less than 100%. Therefore, accurate tones will not be reproduced on a screen.
FIG. 2 is a schematic diagram of an example of the PIF signal and picture signal showing this folding back. An original picture signal 30 is an envelope curve that traces mutually same-phase peaks of the PIF signal 32. A PIF signal 32 has a polarity that is mutually inverted in a period 34 where the degree of picture modulation exceeds 100% and in periods 36 of less than 100%. When the PIF signal 32 is input to the APC wave detector 8, the PLL reflects the 180° phase shift caused by this inversion. As a result, the phase of the reproduced carrier wave input to the VDET 6 is also shifted 180°, and the envelope curve on the lower side indicated by the dotted line is detected as the picture signal 38 in the period 34. Thus, the picture signal having a degree of picture modulation that exceeds 100% is returned downward with the 100% line as a center. Therefore, in this portion, as the degree of picture modulation increases, the picture becomes increasingly unnatural and dark.
In order to avoid this folding back, the conventional circuit is configured so that the APC wave detector 8 will switch the output current IAPC during overmodulation to an extremely low value or to zero on the basis of the results of the assessment performed by the comparator 22. Accordingly, loop gain in the PLL is minimized, the PLL is prevented from also being shifted in phase by 180° when the polarity of the PIF signal 32 inverts, and instances of returning are prevented. In other words, the conventional VIF circuit reduces instances of loop gain during a state of overmodulation, whereby the relationship between the phases of the PIF signal and the reproduced carrier wave is maintained while the PLL is locked before the state of overmodulation occurs. However, when loop gain is minimized, the oscillation of the VCO 10 may be in a state of free run, and it may become impossible to stop drifting in the oscillation frequency of the VCO 10 due to, e.g., changes in the control voltage of the VCO 10 that result from, e.g., electrical discharge in the APC filter 14. For this reason, the relationship between the phases of the PIF signal and the reproduced carrier wave cannot necessarily be suitably maintained in a state of overmodulation. As a result, a problem is presented in that the phase of the reproduced carrier wave provided to the VDET 6 will fluctuate, the precision of the picture signal demodulated in the VDET 6 will decrease, and the picture quality will accordingly deteriorate.